Hi and Welcome to YT!
My Duncan was under 3 pounds at 14 wks and now at 4 yrs old he is 6 pounds and to me this is still small, but I also have a 3 yr old boy Bear that is also lil under 6 pounds but totally different body that you would never think this two were the same weight.
So weight is just a number they all carry it differently.... or at least thats what I keep telling myself....lol
Don't worry about the weight there are some Beautiful over stander size yorkies here, I myself love to look at those threads were YT members show pictures of their larger yorkies.
The main thing is that you have a Healthy puppy and if you do more reading around here you will see why this is the main thing you want out of any pet you love.
West Wishes,